Ensure a class only has one instance, and provide a global point of access to it. Pattern has creational purpose and deals with object relationships, which are more dynamic. The Singleton is often used as a part another design patterns (see Façade and Flyweight).
- there must be exactly one instance of a class, and it must be accessible to clients from a well-known access point
- when the sole instance should be extensible by subclassing, and clients should be able to use an extended instance without modifying their code